With XP it's less hassle to use IDE emulation mode since the AHCI (SATA) drivers aren't included - you'd have to use the "F6 to load drivers" procedure during the XP install and have the AHCI driver disk handy. That's not necessary if the motherboard emulates IDE.
With Vista/Windows 7 AHCI is supported out of the box, so there's absolutely no reason to use IDE emulation mode.
If I use the AHCI driver, which I have on floppy already, it gives me a choice between X86 driver and if I remember correctly X64 driver... Doesn't this choice depend on the version of XP I'm running... or does it depend on the processor?